1. Field of the Invention
The present invention relates to a polyallylene sulfide nonwoven fabric. More particularly, the present invention relates to a nonwoven fabric composed of extra fine fibers of a polyallylene sulfide (hereinafter referred to as PPS) having superior heat resistance, wet heat resistance, resistance to chemicals, and fire retardant properties, and suitable for use in various filter applications.
2. Description of Related Art
PPS fibers can be used as a nonwoven fabric in various applications, for example, filters, fire retardant electrical insulating materials, battery separators or the like, because of its excellent heat resistance, wet heat resistance, resistance to chemicals and fire retardant properties.
A method of obtaining from a resin, a filament having a high-modulus is disclosed in Japanese Examined Patent Publication (Kokoku) No. 52-30609 (corresponding to G.B No. 1420176). The melt flow rate of the resin is lowered by partially precuring the PPS resin to give the PPS resin a high-molecular weight. Japanese Unexamined Patent Publication (Kokai) No. 58-31112 discloses a high speed winding method by which a fiber having a low crystallizing temperature and a high melting point can be obtained.
Further Japanese Unexamined Patent Publication (Kokai) No. 57-16954 discloses a PPS fiber nonwoven fabric manufactured by a spun-bond method in which a flow of a plurality of filaments in a high speed air current is used to obtain a filament web. The filaments are interlaced by a needle punching method. Japanese Unexamined Patent Publication (Kokai) No. 61-289162 disclosed a nonwoven fabric manufactured by heat-fusing a web comprising a blend of heat-resistant fibers and undrawn PPS fibers.
A basic method and apparatus for carrying out a melt-blow process is disclosed in Industrial and Engineering Chemistry, Volume 48, No. 8 published in 1956, from page 1342 to page 1346. Further, a method for manufacturing an extra fine fiber of a polyolefin, a polyester or the like is disclosed in Japanese Examined Patent Publication (Kokoku) No 56-33511 and Japanese Unexamined Patent Publication (Kokai) No. 55-142757. Nevertheless, a nonwoven fabric comprising extra fine fibers of PPS resin is not disclosed in the above references, and heretofore was unknown.
To obtain an efficient filter that can be used under severe conditions of temperature, resistance to chemicals or the like, the fibers constituting the filter must have a high resistance to heat and a high resistance to chemical, in addition to a satisfactory collecting efficiency and pressure loss. To improve the latter requirements, i.e., collecting efficiency and pressure loss, preferably the fiber used is extra fine, and more particularly, the filter is composed of a nonwoven fabric in which the extra fine fibers are randomly arranged.
Although a PPS fiber capable of satisfying the requirements of high heat resistance and high resistance to chemicals is known, a nonwoven fabric made of extra fine fibers and capable of satisfying the latter requirement referred to above have not been obtained from the previously described PPS fiber.
Because PPS resin has a high melting point, a high crystallizability, and a high modulus, it is very difficult to manufacture a fiber having consistant quality or performance by known conventional melt spinning technique used for general fiber-making from polymers such as polyesters, polyamides, polyolefins or the like. In particularly, it has not been possible to manufacture a nonwoven fabric made of PPS fibers having a diameter of 10 .mu.m or less.
Further, since PPS fibers have high stiffness and are brittle, it is extremely difficult to apply to PPS fiber a mechanical crimp as is usually used in processes of manufacturing nonwoven fabric of staple fibers. Additionally, since static electrical properties of PPS fibers are basically high, it is difficult to manufacture a PPS nonwoven fabric by the conventional techniques employed for manufacturing nonwoven fabrics of staple fibers.
Since the nonwoven fabric of the PPS filament disclosed in the Japanese Unexamined Patent Publication (Kokai) No. 57-16954 is manufactured by a spun-bond method in which the formation of a fiber is accompanied by a cool air current, the minimum diameter of the obtained fiber is limited. By the process of this publication, it is impossible to obtain a fiber having a diameter of 10 .mu.m or less. Further, to obtain the disclosed nonwoven fabric, it is necessary to electrostatically disperse the fibers to form a web, to interlace the fibers in the web by needle punching, and to shrink the web at a high temperature. This process of manufacturing a nonwoven fabric takes a long time, and thus this manufacturing method is economically disadvantageous. The biggest problem caused by the relatively large diameter of the fiber is that, when the nonwoven fabric is used for a filter, superior filtering characteristics cannot be obtained. Therefore, the use or application of filters made from such fibers is limited.
The nonwoven fabric disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-289162 is formed by heat-fusing the fibers in a web. Because there is little or no freedom of movement of the fibers in the web, the nonwoven fabric becomes a hard plate. In particular, this nonwoven fabric has low tear strength. Further, the bulk density of the nonwoven fabric is very large. Thus, when this nonwoven fabric is used for a filter, the pressure loss is great. Furthermore, since a carding machine or the like must be used when manufacturing a web by this method, fibers having a thickness of 1 denier or more, i.e., a diameter of about 10 .mu.m or more, must be used. The handling of the nonwoven fabric becomes more hard due to large diameter of the fiber and the filtering characteristics of the nonwoven fabric are not outstanding. Also, the process for manufacturing this nonwoven fabric has many steps, i.e., cutting the fiber, crimping the cut fiber, forming the web from the crimped fiber, and heat fusing the web. Thus this process is disadvantageous from an industrial production viewpoint.